Known analog voltage comparators are often constructed using a differential pair of transistors in which the voltages to be compared are each supplied to a different input transistor. One limitation of this configuration is that there is an inherent offset error due to the unavoidable deviation in the characteristics of different transistors (i.e., transistor mismatch). This offset error often amounts to about 10 mV, which is too large for many applications. For example, if one volt represents 8 bits of data yielding 256 distinct states, only about 4 mV separates adjacent states. Clearly an error greater than 4 mV is often intolerable.
Comparators are known which compensate for the offset error by providing a compensating voltage, but these comparators require additional circuitry and hence are unduly complex. Examples of such comparators are shown in E. A. Vittoz, "Dynamic Analog Techniques," VLSI Circuits for Telecommunications, edited by Y. P. Tsividis and P. Antognetti, Prentice Hall, 1995.